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<title>GNU Compiler Collection (GCC) Internals: OpenMP</title>
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<a name="OpenMP"></a>
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<p>
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Next: <a href="OpenACC.html#OpenACC" accesskey="n" rel="next">OpenACC</a>, Previous: <a href="Cleanups.html#Cleanups" accesskey="p" rel="prev">Cleanups</a>, Up: <a href="Statements.html#Statements" accesskey="u" rel="up">Statements</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="OpenMP-1"></a>
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<h4 class="subsection">10.7.7 OpenMP</h4>
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<a name="index-OMP_005fPARALLEL"></a>
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<a name="index-OMP_005fFOR"></a>
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<a name="index-OMP_005fSECTIONS"></a>
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<a name="index-OMP_005fSINGLE"></a>
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<a name="index-OMP_005fSECTION"></a>
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<a name="index-OMP_005fMASTER"></a>
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<a name="index-OMP_005fORDERED"></a>
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<a name="index-OMP_005fCRITICAL"></a>
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<a name="index-OMP_005fRETURN"></a>
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<a name="index-OMP_005fCONTINUE"></a>
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<a name="index-OMP_005fATOMIC"></a>
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<a name="index-OMP_005fCLAUSE"></a>
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<p>All the statements starting with <code>OMP_</code> represent directives and
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clauses used by the OpenMP API <a href="http://www.openmp.org/">http://www.openmp.org/</a><!-- /@w -->.
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</p>
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<dl compact="compact">
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<dt><code>OMP_PARALLEL</code></dt>
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<dd>
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<p>Represents <code>#pragma omp parallel [clause1 … clauseN]</code>. It
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has four operands:
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</p>
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<p>Operand <code>OMP_PARALLEL_BODY</code> is valid while in GENERIC and
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High GIMPLE forms. It contains the body of code to be executed
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by all the threads. During GIMPLE lowering, this operand becomes
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<code>NULL</code> and the body is emitted linearly after
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<code>OMP_PARALLEL</code>.
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</p>
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<p>Operand <code>OMP_PARALLEL_CLAUSES</code> is the list of clauses
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associated with the directive.
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</p>
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<p>Operand <code>OMP_PARALLEL_FN</code> is created by
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<code>pass_lower_omp</code>, it contains the <code>FUNCTION_DECL</code>
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for the function that will contain the body of the parallel
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region.
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</p>
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<p>Operand <code>OMP_PARALLEL_DATA_ARG</code> is also created by
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<code>pass_lower_omp</code>. If there are shared variables to be
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communicated to the children threads, this operand will contain
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the <code>VAR_DECL</code> that contains all the shared values and
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variables.
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</p>
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</dd>
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<dt><code>OMP_FOR</code></dt>
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<dd>
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<p>Represents <code>#pragma omp for [clause1 … clauseN]</code>. It has
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six operands:
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</p>
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<p>Operand <code>OMP_FOR_BODY</code> contains the loop body.
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</p>
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<p>Operand <code>OMP_FOR_CLAUSES</code> is the list of clauses
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associated with the directive.
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</p>
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<p>Operand <code>OMP_FOR_INIT</code> is the loop initialization code of
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the form <code>VAR = N1</code>.
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</p>
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<p>Operand <code>OMP_FOR_COND</code> is the loop conditional expression
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of the form <code>VAR {<,>,<=,>=} N2</code>.
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</p>
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<p>Operand <code>OMP_FOR_INCR</code> is the loop index increment of the
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form <code>VAR {+=,-=} INCR</code>.
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</p>
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<p>Operand <code>OMP_FOR_PRE_BODY</code> contains side-effect code from
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operands <code>OMP_FOR_INIT</code>, <code>OMP_FOR_COND</code> and
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<code>OMP_FOR_INC</code>. These side-effects are part of the
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<code>OMP_FOR</code> block but must be evaluated before the start of
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loop body.
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</p>
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<p>The loop index variable <code>VAR</code> must be a signed integer variable,
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which is implicitly private to each thread. Bounds
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<code>N1</code> and <code>N2</code> and the increment expression
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<code>INCR</code> are required to be loop invariant integer
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expressions that are evaluated without any synchronization. The
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evaluation order, frequency of evaluation and side-effects are
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unspecified by the standard.
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</p>
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</dd>
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<dt><code>OMP_SECTIONS</code></dt>
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<dd>
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<p>Represents <code>#pragma omp sections [clause1 … clauseN]</code>.
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</p>
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<p>Operand <code>OMP_SECTIONS_BODY</code> contains the sections body,
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which in turn contains a set of <code>OMP_SECTION</code> nodes for
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each of the concurrent sections delimited by <code>#pragma omp
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section</code>.
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</p>
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<p>Operand <code>OMP_SECTIONS_CLAUSES</code> is the list of clauses
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associated with the directive.
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</p>
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</dd>
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<dt><code>OMP_SECTION</code></dt>
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<dd>
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<p>Section delimiter for <code>OMP_SECTIONS</code>.
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</p>
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</dd>
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<dt><code>OMP_SINGLE</code></dt>
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<dd>
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<p>Represents <code>#pragma omp single</code>.
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</p>
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<p>Operand <code>OMP_SINGLE_BODY</code> contains the body of code to be
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executed by a single thread.
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</p>
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<p>Operand <code>OMP_SINGLE_CLAUSES</code> is the list of clauses
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associated with the directive.
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</p>
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</dd>
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<dt><code>OMP_MASTER</code></dt>
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<dd>
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<p>Represents <code>#pragma omp master</code>.
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</p>
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<p>Operand <code>OMP_MASTER_BODY</code> contains the body of code to be
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executed by the master thread.
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</p>
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</dd>
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<dt><code>OMP_ORDERED</code></dt>
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<dd>
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<p>Represents <code>#pragma omp ordered</code>.
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</p>
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<p>Operand <code>OMP_ORDERED_BODY</code> contains the body of code to be
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executed in the sequential order dictated by the loop index
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variable.
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</p>
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</dd>
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<dt><code>OMP_CRITICAL</code></dt>
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<dd>
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<p>Represents <code>#pragma omp critical [name]</code>.
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</p>
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<p>Operand <code>OMP_CRITICAL_BODY</code> is the critical section.
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</p>
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<p>Operand <code>OMP_CRITICAL_NAME</code> is an optional identifier to
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label the critical section.
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</p>
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</dd>
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<dt><code>OMP_RETURN</code></dt>
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<dd>
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<p>This does not represent any OpenMP directive, it is an artificial
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marker to indicate the end of the body of an OpenMP. It is used
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by the flow graph (<code>tree-cfg.c</code>) and OpenMP region
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building code (<code>omp-low.c</code>).
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</p>
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</dd>
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<dt><code>OMP_CONTINUE</code></dt>
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<dd>
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<p>Similarly, this instruction does not represent an OpenMP
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directive, it is used by <code>OMP_FOR</code> (and similar codes) as well as
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<code>OMP_SECTIONS</code> to mark the place where the code needs to
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loop to the next iteration, or the next section, respectively.
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</p>
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<p>In some cases, <code>OMP_CONTINUE</code> is placed right before
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<code>OMP_RETURN</code>. But if there are cleanups that need to
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occur right after the looping body, it will be emitted between
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<code>OMP_CONTINUE</code> and <code>OMP_RETURN</code>.
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</p>
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</dd>
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<dt><code>OMP_ATOMIC</code></dt>
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<dd>
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<p>Represents <code>#pragma omp atomic</code>.
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</p>
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<p>Operand 0 is the address at which the atomic operation is to be
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performed.
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</p>
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<p>Operand 1 is the expression to evaluate. The gimplifier tries
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three alternative code generation strategies. Whenever possible,
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an atomic update built-in is used. If that fails, a
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compare-and-swap loop is attempted. If that also fails, a
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regular critical section around the expression is used.
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</p>
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</dd>
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<dt><code>OMP_CLAUSE</code></dt>
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<dd>
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<p>Represents clauses associated with one of the <code>OMP_</code> directives.
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Clauses are represented by separate subcodes defined in
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<samp>tree.h</samp>. Clauses codes can be one of:
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<code>OMP_CLAUSE_PRIVATE</code>, <code>OMP_CLAUSE_SHARED</code>,
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<code>OMP_CLAUSE_FIRSTPRIVATE</code>,
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<code>OMP_CLAUSE_LASTPRIVATE</code>, <code>OMP_CLAUSE_COPYIN</code>,
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<code>OMP_CLAUSE_COPYPRIVATE</code>, <code>OMP_CLAUSE_IF</code>,
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<code>OMP_CLAUSE_NUM_THREADS</code>, <code>OMP_CLAUSE_SCHEDULE</code>,
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<code>OMP_CLAUSE_NOWAIT</code>, <code>OMP_CLAUSE_ORDERED</code>,
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<code>OMP_CLAUSE_DEFAULT</code>, <code>OMP_CLAUSE_REDUCTION</code>,
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<code>OMP_CLAUSE_COLLAPSE</code>, <code>OMP_CLAUSE_UNTIED</code>,
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<code>OMP_CLAUSE_FINAL</code>, and <code>OMP_CLAUSE_MERGEABLE</code>. Each code
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represents the corresponding OpenMP clause.
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</p>
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<p>Clauses associated with the same directive are chained together
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via <code>OMP_CLAUSE_CHAIN</code>. Those clauses that accept a list
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of variables are restricted to exactly one, accessed with
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<code>OMP_CLAUSE_VAR</code>. Therefore, multiple variables under the
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same clause <code>C</code> need to be represented as multiple <code>C</code> clauses
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chained together. This facilitates adding new clauses during
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compilation.
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</p>
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</dd>
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</dl>
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<hr>
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Next: <a href="OpenACC.html#OpenACC" accesskey="n" rel="next">OpenACC</a>, Previous: <a href="Cleanups.html#Cleanups" accesskey="p" rel="prev">Cleanups</a>, Up: <a href="Statements.html#Statements" accesskey="u" rel="up">Statements</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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